Eddy current sensors are known and used in a variety of applications. One use of eddy current sensors is monitoring for defects in turbine blades of a turbine engine. As a turbine blade spins in a turbine engine, the blade is affected by centrifugal forces and vibrations, which may induce material defects in the turbine blade. As material defects grow, the length of the turbine blade may increase and, in some cases, collide with the inner diameter of the turbine casing, causing damage to the turbine engine.
In a typical eddy current application, the eddy current sensor is typically placed within the turbine casing, with the closest distance between the tip of the turbine blade and the eddy current sensor typically referred to as the standoff distance. An output signal from the eddy current sensor is correlated to the standoff distance such that any change in the standoff distance is reflected in a change in the output signal. By comparing multiple measurements of the eddy current signal and the standoff distance, the growth of a turbine blade or other changes in the turbine blade or blade hub may be sensed. By detecting abnormal growth or changes in the turbine blades early, the turbine blades may be replaced or repaired before expensive damage occurs.
FIG. 5 illustrates a conventional eddy current sensor module 500. As shown in FIG. 5, the eddy current sensor module 500 includes an eddy current sensor 510 and an electronic board 520.
The eddy current sensor 510 may be an eddy current sensor such as described in U.S. Pat. No. 5,942,893 (issued Aug. 24, 1999 and common assignee) to Terpay, which is hereby incorporated by reference in its entirety. Alternatively, other eddy current sensors may be obtained from GMW Associates, W. C. Branham, Globalspec or other similar situated vendor. The eddy current sensor 510 is configured to generate a voltage waveform signal as the turbine blade 540 approaches, passes and departs from the eddy current sensor.
Returning to FIG. 5, the voltage signal from the output eddy current sensor 510 may be outputted to the electronic board 520. The electronic board 520 may be configured to supply power to the eddy current sensor 520 with a radio-frequency (RF) amplifier operating, for example, at 15 MHz. The electronic board 520 may also be configured to receive signals from the eddy current sensor 510 and mix the received signal with the frequency of the RF amplifier. The raw analog signal from the mixer is amplified with another amplifier and filtered to generate the eddy current sensor baseband waveform, which is shown in FIG. 6.
FIG. 6 illustrates a baseband waveform 600 from the eddy current sensor 510 for a passing target, e.g., a turbine blade, as processed by the electronic board 520. As shown in FIG. 6, the baseband waveform 600 is plotted on a time versus voltage graph. The baseband waveform 600 includes two peak voltages, V+peak:T1 and V−peak:T1. These two peak values are the largest voltage values, which occur at t+peak and t−peak, respectively. The baseband waveform 600 includes two waveform parameters, the peak-to-peak voltage between V+peak:T1 and V−peak:T1 and the zero crossing slope at time, t0. The slope of the waveform signal at this particular point is characterized by m0. The standoff distance between a conductive element and an eddy current sensor may be determined as an eddy current sensor generated waveform signal as a function of the peak-to-peak voltage waveform parameter.
In constant temperature conditions, the amplitude of V+peak and V−peak, or the peak-to-peak voltage, increases with a decrease in standoff distance. Unfortunately, the signal produced by the eddy current sensor is impacted by and varies with the temperatures of the eddy current sensor as well as the temperature of the electronics components on the signal conditioning circuit board. Generally, as the temperature rises, the signal diminishes for a fixed standoff distance between the eddy current sensor and the target, as shown in FIG. 6. FIG. 6 includes three waveforms generated at a fixed standoff distance at three different temperatures where T1<T2<T3.
In operation, the eddy current sensors are often exposed to extreme environments like the interior of a turbine engine with varying operating temperatures under different operating and flight conditions. As such, the waveforms generated by the eddy current sensors may not only vary with changes in the actual standoff distance between the eddy current sensors and the turbine blades, but may also vary with the changes in temperature experienced by the eddy current sensor. Additionally, the waveform is also impacted by the temperature of the electronics and wiring used to drive, sense and record the signal generated by the eddy current sensor. Therefore, the recorded eddy current signal may not accurately reflect the true standoff distance because of the impact the temperature of the eddy current sensor and the temperature of the sensor electronics may have on the signal.
Unfortunately, changes in the waveform due to rising changes in the temperature of the sensor and sensor electronics may suggest that the standoff distance is growing, when in fact the standoff distance shrinking. Such data may mislead system operators and maintenance crews into a false sense of security until extensive and costly damage occurs in the aircraft engine. Conversely, lowing changes in the temperature may have the opposite effect, causing maintenance crews to prematurely replace expensive turbine blades, resulting in the waste of healthy turbine blades and the costly waste of money and flying time for the aircraft.
Therefore, there exists a need to compensate for the impact of temperature on the waveform signal and the standoff distance determination so as to improve the accuracy of the collected standoff distance data and the reliability of rotating machinery.